With the advent of multimedia communications, there arises the need for low-cost solid state imagers to complement communication devices and computers. An image input sensor is central to any teleconferencing and multimedia application and is used to convert an optical image focused on the sensor into electrical signals. The image input sensor typically includes an array of light detecting elements where each element produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or to help record information about the optical image, as performed by a digital camera.
A common type of image sensor is a charge coupled device (CCD). CCDs have dominated vision applications because of their superior dynamic range, low fixed-pattern noise and high sensitivity to light. However, CCD technology is quite complex, suffers from low yields, and is expensive due to specialized processing involved to produce such devices. Moreover, integrating analog/digital convertors and digital processing with a CCD array on the same chip is not feasible. As a result, multiple chip sets are required to accommodate the CCD array image sensor and the required logic and processing circuitry. An imaging sensing device having a multiple chip set package is more costly to produce than a device utilizing a single chip set. Other well known disadvantages exist for CCDs.
In comparison, image sensor research has advanced pure complementary metal oxide semiconductor (CMOS) image sensors. CMOS image sensors consist of photodiodes or phototransistors that are used as the light detecting elements where the conductivity of the elements correspond to the intensity of light impinging on the elements. CMOS image sensors have a great cost advantage over CCDs of similar resolution. This is due to the fact that CMOS image sensors have high yields because they are fabricated by the same semiconductor foundries that make computer memory chips, digital signal processors, analog/digital convertors, etc.
CMOS image sensors address many of the shortcomings of CCDs. CMOS technology allows the fabrication of a single chip set having image capture capability along with logic and processing capabilities including analog/digital conversion and digital signal processors. Memory such as RAM and ROM can also be integrated onto the same single chip set. However, integrating non-volatile memory onto the same circuit as a CMOS imager presents a problem due to the non-volatile memory's sensitivity to light. Most non-volatile memory cells rely on a trapped charge stored on a floating gate in a field effect transistor (FET). Erasure of the trapped charge from the floating gate is performed by exposure to ultraviolet light.
As a consequence, current image devices, such as digital cameras, do not place non-volatile memory on the same integrated circuit as the CMOS image sensor. Separating the CMOS imager from non-volatile memory results in an image sensing device having a multiple chip set package which adds to the cost of the device. In addition, the size of the image sensing device is directly related to the required number of chip sets. If non-volatile memory were integrated onto the same integrated circuit as a CMOS imager, then a more compact image sensing device could be achieved.
Therefore, what is needed is a single integrated circuit having a CMOS imager and peripheral components for receiving and processing a received image, including non-volatile memory integrated onto the same integrated circuit. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to provide a CMOS imager with integrated non-volatile memory wherein light received by the image sensor does not effect the non-volatile memory.